There is a considerable body of experimental evidence that metal binding may be implicated in the antibiotic mode of action of drugs of the tetracycline series. These drugs have a number of potential metal binding sites, and drug-metal complexes are known. However the site at which a given metal ion is bound to the drug molecule has not been firmly established. It is proposed that a thorough investigation of the site of binding in both therapeutically active and inactive tetracyclines be carried out for a variety of metal ions. Experimental work will involve examining perturbations of proton nuclear magnetic resonance spectra of the drugs when small amounts of paramagnetic metal ions are present. Analysis of these perturbations should reveal the site or sites of metal binding. It is hoped that a better understanding of the mode of tetracycline drug action will be gained from a comparison of the metal binding characteristics of active and inactive tetracycline derivatives.